Hopes for finding life on four rocky exoplanets relatively close to Earth have been boosted by new modelling that shows biological systems could survive the intense and prolonged bursts of X-ray and ultraviolet (UV) radiation to which they are subject.
The analysis, by astronomers Lisa Kaltenegger and Jack O’Malley-James, both from Cornell University in the US, suggests life could sustain in a wide range of planetary atmospheres, not only those roughly analogous to that found on Earth, but also eroded and thin types devoid of oxygen and protective ozone.
The work is published in the journal Monthly Notices of the Royal Astronomical Society.
The four closest rocky exoplanets known to be orbiting in the habitable zone (HZ) of their host stars are Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b.
Of these, the closest to Earth is Proxima-b, a planet with a mass about 1.3 times larger than that of the Earth, orbiting the star Proxima Centauri about 4.2 light-years from the sun.
Proxima Centauri, in common with the hosts of the other three candidate planets, is a red dwarf – a star much smaller, cooler, and longer lived than the sun. Also known as M-stars, red dwarfs are by far the most common type of star in the universe.
They also throw the cosy concept of habitable zones into a cocked hat. The region, also known as the Goldilocks Zone, is defined as the orbital distance from a star that is far enough away, but also close enough in, to maintain water in a liquid state.
Because red dwarfs are much cooler than larger stars, the nominal habitable zone is relatively closer in – and this means that the planets in it are bombarded by huge amounts of X-ray and ultraviolet radiation that emanate from the star.
Proxima-b, for example, receives 250 times more X-ray radiation than Earth.
Several studies have concluded that the violence of such exposures would prevent the start of any biotic processes. Huge incoming UV fluxes would also likely boil off any liquid water.
Indeed, in a 2017 study, Kaltenegger and O’Malley-James published a paper that delivered a notably pessimistic assessment of the chances of life existing on another of the new paper’s target exoplanets, TRAPPIST-1e.
An ozone-poor atmosphere around the TRAPPIST-1 system planets, they concluded, would result in “surface environments hostile even to highly UV tolerant terrestrial extremophiles”.
Their latest research, however, finds them adopting a much more positive tone.
Life on all four target planets is distinctly possible, they conclude. And they reach that conclusion by using what is usually the bane of astrobiology – the fact that thus far there is only one known biologic system: Earth’s.
As Arizona State University astrobiologists Sara Imari Walker and Paul Davies have pointed out, having essentially a sample size of one makes it impossible to distinguish between structures that might be unique artefacts of the Earth system, and those that might arise from universal “law-like” processes.
Kaltenegger and O’Malley-James, however, avoid this tricky issue by looking into the deep history of the planet.
Four billion years ago, when life began, they point out, Earth’s atmosphere was very different, and the planet was subjected to an enormous and extremely powerful influx of radiation.
And yet, life emerged.
Their modelling of possible atmospheric conditions on the target exoplanets results in good news, even in worst-case scenarios.
“While the anoxic atmosphere does result in a considerably more biologically harmful radiation environment compared to the present-day Earth, it is still approximately an order of magnitude less biologically harmful than early Earth’s,” they write.
“Therefore, UV surface radiation levels should not rule out surface habitability for our closest potentially habitable planets or for planets orbiting in the HZ of active M stars in general.”
High UV scenarios, they note, may last for many billions of years, given the extreme longevity of red dwarfs, and may thus lead to the evolution of detectable traits.
“In a high UV surface environment, mechanisms that protect biota from such radiation can play a crucial role in maintaining surface habitability, especially on planets around active M stars with thin, eroded or anoxic atmospheres, where other UV-attenuating gases/particles are not present,” they write.
To bolster the assertion the researchers offer examples drawn from terrestrial biology. Some species of microorganisms and lichens, for instance, have been taken into space on various missions and have survived full solar UV exposure, deploying protective cells or pigments as UV screens.
Living beneath thin layers of sand or soil also reduces UV exposure, and the shortest UV wavelengths can be reduced by an order of magnitude by as little as one micrometre of water.
Kaltenegger and O’Malley-James concede that knowing the precise atmospheric conditions that pertain on Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b is currently impossible.
However, they suggest that although UV radiation may not be the only factor affecting the chances of life arising, it should not be considered by itself enough to reduce those chances to zero, however tough conditions might be.
“Even for planets with eroded or anoxic atmospheres orbiting active, flaring M stars the surface UV radiation in our models remains below that of the early Earth for all cases modelled,” they conclude.
“Therefore, rather than ruling these worlds out in our search for life, they provide an intriguing environment for the search for life and even for searching for alternative biosignatures that could exist under high-UV surface conditions.”
